Alkaline hydrolysis of photovoltaic backsheet containing PET and PVDF for the recycling of PVDF

Recovering fluorine from end-of-life products is crucial for the sustainable production and consumption of fluorine-containing compounds because fluorspar, an important natural resource for fluorine, is currently at a supply risk. In this study, we investigated the feasibility of chemically recycling a fluorine-containing photovoltaic (PV) backsheet for fluoropolymer recycling. Herein, a PV backsheet consisting of laminated polyethylene terephthalate (PET) and polyvinylidene fluoride (PVDF) was treated with different concentrations of sodium hydroxide (NaOH) to hydrolyze the PET layer to water-soluble sodium terephthalate (Na2TP) and to separate pure PVDF layer as a solid material. Optimized alkaline conditions (up to 10 M NaOH at 100 °C for 2 h) were determined, under which 87% of the PET layer could be decomposed without any significant deterioration of the PVDF layer. The hydrolysis kinetics of PET layer in NaOH could be explained by the modified shrinking-core model. Considering that the mass of end-of-life PV panels in Japan is estimated to increase to approximately 280,000 tons per year by 2036, PV backsheets are attractive candidates for fluoropolymer recycling, which can be effectively achieved using chemical recycling approach demonstrated in this study.


Introduction
Fluorine has the highest electronegativity and strongly binds to various elements, especially to carbon as the C-F bond has a short bond distance and high bond energy. Hence, fluorine-containing compounds exhibit properties such as chemical, heat, and weather resistance, and non-adhesiveness. These properties enable their usage as physiologically active substances in the medical and agricultural industries and as functional materials in the chemical and electronics industries, indicating that fluorine has become an indispensable resource in our daily life [1]. With the recent spread of electronic devices, such as lithium-ion batteries and photovoltaic (PV) modules, and the development of communication infrastructure and energy industries, the global demand for fluorine is expected to steadily increase [2].
Pure fluorine does not exist in nature and fluorine is typically found in minerals, such as fluorspar, cryolite, and apatite [3]. Fluorspar is widely used commercially as a raw material for fluorine. In the supply chain, the extracted fluorspar is refined and used in the manufacture of hydrogen fluoride, which is the starting material for various fluorinecontaining compounds such as fluorocarbons and fluoropolymers. USGS Mineral Commodity Summaries reported that the global fluorspar mine production in 2021 was 8.6 million tons and its reserve was estimated to 320 million tons [4]. However, the fact that the global supply of fluorspar is at risk has been acknowledged. The European Commission periodically reports the list of Critical raw materials, which is the list of indispensable raw materials that are economically important and whose global supply is at risk. Fluorspar has perennially remained on the list because its production is limited to specific countries [5]. The concentration ratio of the top 3 fluorspar-producing countries was 85% in 2020 [4]. Moreover, most fluorine-containing compounds are disposed of in landfills as waste and its end-of-life recycling input rate is only 1% [6,7]. Previously, industrialized countries such as the United States and Japan and those in Europe mined fluorspar in their own countries. However, these countries currently depend on imports from other countries for fluorine procurement. Therefore, promoting fluorine recovery from waste will reduce the risk of fluorine supply and enhance the sustainability of domestic industries. PV backsheets are attractive candidates for fluorine recovery. Depending on the type of semiconducting material installed in the PV panel, multiple types of PV panels such as monocrystalline, polycrystalline, and thin-film solar panels are in use currently [8]. Figure 1 shows a schematic of the typical structure of a crystalline silicon solar panel. Generally, the backsheet of a crystalline silicon solar panel comprises multilayer laminated fluoropolymers and engineered thermoplastics [9,10]. Polyvinyl fluoride (PVF, DuPont tradename "Tedlar") and polyvinylidene fluoride (PVDF, ARKEMA tradename "Kynar") are the most widely used fluoropolymers in PV backsheets [9,11]. Because both PVF and PVDF are thermoplastic polymers, their waste can be recycled to produce new fluoropolymers via melting and deforming processes in the plastic recycling market [12]. In Japan, PV panels have become widespread since 2012 and the product lifespan is expected to be approximately 20 years [13]. Consequently, the estimated mass of end-oflife PV panels will increase sharply after 2030 and become approximately 280,000 tons per year by 2036. Therefore, the end-of-life PV backsheets could become a major source of fluorine in the next few decades. A recycling scheme for end-of-life PV panels had not yet been established in Japan, but the government of Japan is now actively promoting environmentally sound waste management of these materials [14]. The recycling of end-of-life PV panels is of great significance for effective pollution prevention and resource use, and research to explore the possibility of recycling not only valuable metals but also plastics used in PV panels is necessary.
As shown in Fig. 1, a typical structure of a PV backsheet consists of three layers of laminated plastics-a fluoropolymer, polyethylene terephthalate (PET) and another layer of fluoropolymer, which are bonded to each other. Previous studies have analyzed the deterioration mechanism of PV panels via chemical and thermal treatment, and several recycling technologies have been proposed [15][16][17][18][19][20][21]. However, most of them focused on metal and glass recycling by dismantling PV panels and deteriorating encapsulant. Fiandra et al. [22] demonstrated delamination using a milling machine to separate the plastic from the PV backsheet; however, both fluoropolymer and PET remained mixed after the procedure and were not collected separately. Dias et al. [23] applied electrostatic separation technology to shredded PV panels; however, separation of different types of plastic was not achieved. Therefore, investigations on fluoropolymer separation from PV backsheets have not been conducted.
PET decomposition is one technique to segregate pure fluoropolymers from PV backsheets. PET decomposition has been widely investigated under thermal and chemical conditions. Thermal decomposition of PET occurs in the temperature Fig. 1 Schematic illustrating the typical structure of a crystalline silicon solar panel range 350-500 °C. However, the thermal decomposition of PVDF also progresses within this temperature range and generates toxic hydrogen fluoride gas in the process, making it challenging to recover pure PVDF by thermal decomposition [24,25]. Other decomposition methods such as glycolysis, methanolysis, and hydrolysis have also been investigated for chemical treatment of PET [26]. Glycolysis and methanolysis have the advantage of decomposing PET rapidly, however, these reactions require catalystsas as well as high temperature and high pressure, which complicates the treatment process [27]. Hydrolysis is capable of decomposing PET under acid, neutral, and alkali conditions [28][29][30][31][32]. Many efficient PET decomposition methods under chemical conditions have been proposed, but there are also challenges in commercially implementing the technologies. PET decomposition via acid hydrolysis requires countermeasures against equipment corrosion, and the decomposition in organic solvents requires fire prevention and odor control systems. Therefore, alkaline hydrolysis was selected for this study because the reaction progresses under mild conditions of 100 °C or less and at atmospheric pressure, that is suitable to address the recycling of PV backsheets expected to be discarded in large quantities in the future.
The objective of this study is to examine the applicability of chemical treatment in separating fluoropolymers from endof-life PV backsheets for fluoropolymer recycling. In order to collect pure fluoropolymer, the PET layer of PV backsheets must be decomposed effectively without any deterioration of the fluoropolymer layer. PET hydrolysis by alkaline solution has been studied previously [28][29][30]; however, the applicability of this chemical treatment to PV backsheets remains unclear. To investigate the physical separation of pure fluoropolymer, the backsheet containing PET and PVDF layers was hydrolyzed to convert the PET layer into water-soluble sodium terephthalate (Na 2 TP), thus enabling the filtration of the PVDF layer as a solid residue. The hydrolysis kinetics of PET layer in sodium hydroxide (NaOH) were also analyzed. In addition to the investigation on PET hydrolysis, the effects of the alkaline solution on PVDF were analyzed. Generally, PVDF is highly resistant to both acidic and alkaline conditions; however, in highly alkaline solutions, PVDF deteriorates via dehydrofluorination and C-C double bond formation [33][34][35][36][37][38]. Therefore, determining the optimum conditions for alkaline hydrolysis of the PV backsheet such that the PET layer is effectively hydrolyzed to promote PVDF separation while restricting its deterioration is necessary.

Materials
Multiple end-of-life, crystalline silicon PV panels were provided by a waste management and recycling company in Japan. The surface glass from each panel was removed by shot blasting after physically detaching the aluminum frame. The cross section of each panel was observed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX) (TM4000, Hitachi High-Tech Corporation, acceleration voltage: 15 kV, backscattered electron image, EDX software: The Oxford Instrument AZtec) to determine the presence of fluorine in the PV backsheet and to calculate the thickness of each plastic layer. Next, the fluorine-containing backsheet was manually removed from the panel and Fourier-transform infrared spectroscopy (FT-IR) (Nicolet Summit, Thermo Scientific, attenuated total reflection method, software: OMNIC Specta) was performed to identify the type of fluoropolymers and to confirm PET in the backsheet. The backsheet containing PVDF was then freeze-shredded to a length of 4 mm or less using a cutting mill (SM300, Retsch) to prepare samples for alkaline hydrolysis. The theoretical weight percentage of PET and PVDF in the backsheet was calculated from the densities of PET (1.38 g/cm 3 ) and PVDF (1.76 g/cm 3 ) and the thickness of the sheet as determined from the cross-sectional SEM/ EDX image of the backsheet. To investigate whether the composition ratio of PET and PVDF remained constant before and after shredding, the fluorine content in the samples was analyzed by an automatic quick furnace (AQF-2100H, Nittoseiko Analytech, inlet temperature: 900 °C, outlet temperature: 1,000 °C) and ion chromatography (IC) (Dionex Integrion RFIC, Thermo Scientific, column: Ion-Pac AG19/AS19, suppressor: ADRS-600 4 mm, eluent: 45 mM KOH, eluent flowrate: 1.0 mL/min) for statistical variability measurements.

Alkaline hydrolysis
Shredded backsheet (0.5 g) and 25 mL of NaOH solutions (1, 2.5, 5, and 10 M) were placed in a 300 mL stainless steel reactor. The backsheet was then hydrolyzed by heating the reactor in a silicon oil bath at 100 °C for 2 h under a nitrogen flow of 100 mL/min. To promote the reaction between the shredded backsheet and NaOH solution, baffle plates were installed inside the reactor, which was continuously stirred by rotation at 40 rpm while heating. After the reaction, the reactor was cooled to 25 °C, and the unreacted solids were collected by filtration. The filtrate containing Na 2 TP was neutralized with 10 M H 2 SO 4 to precipitate terephthalic acid (TPA). The precipitated TPA was filtered and dried overnight at 40 °C in a dryer, while the filtrate was stored as a reaction solution for the analysis of fluorine ion concentration.

Analysis of PET decomposition and PVDF deterioration
The weight of the recovered TPA was measured to calculate the PET decomposition rate because of alkaline hydrolysis, defined as follows: where D PET is the decomposition rate of PET, W TPA, r is the weight of the recovered TPA, and W TPA, t is the theoretical weight of TPA units in PET layer. Recovered TPA was analyzed by FT-IR for material identification. The reaction kinetics of PET hydrolysis were analyzed using the modified shrinking-core model.
The unreacted solids were dried after washing with ionexchanged water, and image analysis was performed to observe the PVDF and unreacted PET layers using a digital microscope (VHX-7000, KEYENCE). The structural deterioration of the PVDF layer because of alkaline hydrolysis was analyzed by FT-IR.
Finally, IC was performed on the filtrate obtained after neutralization reaction to investigate the fluorine content, which might have increased due to the dehydrofluorination of the PVDF layer during alkaline hydrolysis. The defluorination rate was defined as follows: where D f is the defluorination rate, W f, filtrate is the weight of fluorine in the filtrate, and W f, backsheet is the weight of fluorine in the shredded backsheet.

Analysis of PV backsheet used for alkaline hydrolysis
backsheet consisted of three layers of plastics, and the fluoropolymers were on the outer sides of the backsheet. The thickness of both fluoropolymers was 30.3 µm, and that of the inner plastic was 260 µm. FT-IR analysis on the backsheet is shown in Fig. S1 in the Supporting Information and it revealed that both sides of the fluoropolymers were PVDF and the inner plastic was PET. The theoretically calculated weight percentages of the PET and PVDF layers of the backsheet were 77.1 and 22.9 wt%, respectively. The fluorine content of the original backsheet was 9.7 ± 0.4 wt% (n = 5), whereas that of shredded backsheet was 9.3 ± 0.3 wt% (n = 5); therefore, the variation of plastic composition between samples originating from the shredding process was not significant. The shredded backsheet used as the samples for alkaline hydrolysis is shown in Fig. 3.

PET decomposition
The analytical results for the recovered TPA, shown in Fig.  S2 in the Supporting Information, verify the recovery of TPA from PET layer via alkaline hydrolysis and neutralization. Figure 4 shows the effects of the alkaline concentration and reaction time on PET decomposition rate (given by Eq. 1). Alkaline hydrolysis with 1 M NaOH solution resulted in low  TPA recovery even after the reaction time was increased, and the PET decomposition rate was 30% upon heating for 2 h. In contrast, in higher concentrations of NaOH, the PET hydrolysis proceeded effectively. PET decomposition rate after heating for 2 h in 5 and 10 M NaOH solution was 86% and 87%, respectively. Alkaline hydrolysis using 2.5 M NaOH was not completed in the predetermined reaction time. These results indicate that the PET layer of the PV backsheet was hydrolyzed quantitatively according to Eq. 3 [28].
PET decomposition proceeded under milder conditions compared to those of previous studies [28,30] probably because the thickness of the PET layer of the PV backsheet was only 260 µm (Fig. 2a), which enabled facile alkaline hydrolysis within the predetermined time of the experiment. In addition, the alkaline solution reached PET more efficiently than expected because of the partially peeled-off PVDF layer obtained from the shredding process of the sample preparation. Moreover, stirring the reactor during heating may have promoted alkaline hydrolysis. Although alkaline hydrolysis with 5 M and 10 M NaOH solutions efficiently decomposed the PET layer within 2 h, hydrolysis with 1 M and 2.5 M NaOH solutions required longer duration for the completion of the reaction. As previously reported [28,30], the reaction temperature is a key parameter for the reaction efficiency of alkaline hydrolysis; thus, PET decomposition rate at lower alkaline concentrations could be improved by increasing the reaction temperature.
In addition, the PET hydrolysis kinetics were analyzed by the modified shrinking-core model, which expresses the effective surface area for PET hydrolysis using the degree of hydrolyzed PET (X) and a proportional constant (c) because of the formation of pore and cracks on PET during hydrolysis [32]. The growth and formation of pore and cracks is estimated to increase the effective surface area in this model.
where S is the effective surface area and r X is the particle size of PET at any reaction time.
Here, the rate of reaction is expressed as follows: where V is the rate of reaction, k is the rate constant per unit surface area, and C A is NaOH concentration.
If the PET is regarded as a sphere and its mass balance is considered to be solid, then where is the density of PET.
The degree of hydrolyzed PET, X, can be represented as follows: where r 0 is the initial size of PET particle. The following equation can be derived from Eqs. 4-7.
Integrating both sides, we obtain: where K is the apparent rate constant equal to 3kC A ∕r 0 .
The effect of NaOH concentration and reaction time, as predicted by Eq. 9, is shown in Fig. 5. For 10 M NaOH solution, plots were obtained assuming that almost all the PET was decomposed 60 min after the start of the reaction. Straight lines with a correlation coefficient of 0.93 or higher were obtained for PET hydrolysis at any alkaline concentration, indicating that the PET decomposition in this reaction proceeds according to the modified shrinking-core model.

Surface analysis of the backsheet
Microscopic images of the unreacted solids after alkaline hydrolysis for 2 h are shown in Fig. 6. The white PVDF layer covered both sides of the transparent PET layer in the original backsheet; however, a part of the PVDF layer was peeled off from the PET layer after freeze-shredding (Fig. 6a). No significant change was observed in either PET or PVDF of the backsheet hydrolyzed with 1 M NaOH; however, after alkaline hydrolysis in 2.5 M NaOH solution, PET became brittle slightly and physical separation between the PET and PVDF layers was partially observed (Fig. 6b, c). Under higher concentrations of NaOH, while some brittle PET remained as unreacted solids, most of it decomposed, thus enabling the isolation of pure PVDF (Fig. 6d, e). These results demonstrated that more than 80% of PET decomposition would be required to the PV backsheet for the promotion of PVDF separation from PET layer. Although PET decomposition and PVDF separation proceeded efficiently in high alkaline concentrations, surface discoloration of the PVDF layer was observed.

PVDF deterioration after alkaline hydrolysis
The structural changes in PVDF following alkaline hydrolysis were analyzed using FT-IR spectroscopy (Fig. 7). The characteristic bands of PVDF that were identified in the spectrum of the original PVDF layer, including C-H bending (1400 cm −1 ), C-F stretching (1180 cm −1 ), C-H wagging (870 cm −1 ), and C-F bending (830 cm −1 ) [38], were also observed in that of the hydrolyzed PVDF layer. Micrographs of the unreacted solids (Fig. 6) showed a slight color change on the surface of PVDF treated with NaOH solutions of higher concentrations; however, no significant difference was observed in the FT-IR spectra of the original and hydrolyzed PVDF. Similar to the deterioration of polyvinyl  [39][40][41], the deterioration of PVDF proceeds via dehydrofluorination and C-C double bond formation (Eq. 10) [34,37]; therefore, we prepared a deteriorated backsheet by heating shredded backsheet in 10 M NaOH at 130 °C for 6 h to compare the FT-IR spectra. The FT-IR spectrum of the deteriorated PVDF layer exhibited a broad band in the wavenumber range of 1750-1500 cm −1 with its maximum intensity at 1650 cm −1 . This band was attributed to the C-C double bonds formed through the dehydrofluorination reaction, but it did not appear in the spectrum of the PVDF layer hydrolyzed using 10 M NaOH at 100 °C for 2 h. This result suggests that the deterioration of PVDF with significant structural changes might not progress under the alkaline conditions of the experiment.
The rate of defluorination of the PV backsheet via alkaline hydrolysis (given by Eq. 2) for 2 h was 0.27% in 10 M NaOH solution and significantly lower at lower concentrations of NaOH. As the limit of detection (LOD) of the IC conducted in this study was 0.4 μg/L (S/N = 3), fluorine might have been eliminated from PVDF through deterioration although its concentrations in the filtrate was extremely low. This experiment demonstrated that the PVDF layer could be separated from the PV backsheet by alkaline hydrolysis of the PET layer; however, the deterioration of PVDF via the dehydrofluorination reaction proceeds simultaneously with an insignificant structural change occurring in PVDF. (10) Although significant PVDF deterioration was not confirmed, slight color changes were observed on the surface of the PVDF layer. The tolerance for color changes cannot be determined as it will depend on the post-recovery usage of the recovered fluoropolymer. However, if the PVDF layer is not deteriorated, the recovered PVDF would meet the quality requirement for secondary plastics and could be used in the plastic recycling market. As surface color changes were not observed for PVDF treated with solutions containing lower NaOH concentrations, recovered PVDF that is of the same quality as virgin PVDF could be collected from the PV backsheet treated under milder and more optimum conditions that would suppress PVDF deterioration and promote PET decomposition.

Fluoropolymer recycling scheme
Based on the results of this study, we propose a fluoropolymer recycling scheme for end-of-life PV panels (Fig. 8).
Firstly, the PV backsheet should be shredded before alkaline hydrolysis. The shredding process is effective for making smaller the backsheets and increasing the surface area of the PET layer to improve its contact with the alkaline solution for hydrolysis. Then, shredded backsheet is placed into a heated reactor to promote PET decomposition. Alkaline hydrolysis enables the decomposition of the PET layer in the PV backsheet and promotes the physical separation of the PVDF layer from the PET layer via filtration. Highly alkaline conditions promote PET decomposition but also deteriorate PVDF; therefore, moderately alkaline conditions are required to collect pure PVDF without defluorination and any structural deterioration. The PET layer in the PV backsheet could be decomposed in alkaline solutions with concentrations under 10 M NaOH; however, several brittle PET layers were formed as reaction intermediates, as evidenced by digital microscopy. To   Fig. 7 FT-IR spectra of PVDF in PV backsheet address this problem, another approach to separate brittle PET and PVDF might be required to enhance the purity of the recovered PVDF. Sink-float separation techniques based on the differences in the densities of plastics and triboelectrostatic separation based on the differences in surface potential values might be applicable for the later stages of the recycling scheme. As fluoropolymers generally have high alkaline resistance, the suggested recycling schemes are applicable to all PV backsheets containing PVDF and other types of fluoropolymers.

Conclusion
This study demonstrated that the PVDF layer in the PV backsheet could be separated as pure PVDF via alkaline hydrolysis of the PET layer. Alkaline hydrolysis enables the conversion of PET into water-soluble materials, whereupon the isolated pure PVDF can be collected by filtration of the reaction solutions. The alkaline hydrolysis kinetics of PET are explained by the modified shrinking-core model. Increasing the alkali concentration and reaction time improved the efficiency of PET decomposition. However, PVDF deteriorates under highly alkaline conditions. Therefore, we determined the optimum reaction conditions for hydrolyzing the PET layer efficiently and promoting the separation of the PVDF layer while restricting its deterioration. Our findings indicated that the structure of PVDF does not deteriorate significantly at NaOH concentrations of up to 10 M at 100 °C for 2 h. Therefore, chemical treatment is effective at separating fluoropolymers from end-of-life PV backsheets. Fluoropolymer recycling could be achieved by melting and extruding the recovered fluoropolymers, which in turn could be used to produce new fluoropolymers. Furthermore, we proposed a potential fluoropolymer recycling scheme from end-of-life PV backsheets. Plastic recycling from PV panels has rarely been reported, but our scheme could enhance the recycling of fluoropolymers.
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